If you do not have Composer installed, please consult the official docs.

Once you have installed the library, you will need to load Composer’s autoloader
(which registers all the required namespaces). To do this, place the following
line of PHP code at the top of your application’s PHP files:

require'vendor/autoload.php';

This assumes your application’s PHP files are located in the same folder as
vendor/. If your files are located elsewhere, please supply the path to
vendor/autoload.php in the require statement above.

If you have specific problems or bugs with this SDK, please file an issue on
our official Github. We also
have a mailing list,
so feel free to join to keep up to date with all the latest changes and
announcements to the library.